Abandoned Mines and Naturally Occurring Acid Rock Drainage on National Forest System Lands in Colorado Matthew A. Sares1, Daryl L. Gusey2, and John T. Neubert1 ABSTRACT The Colorado Geological Survey completed an inventory of environmental degradation associated with abandoned and inactive mines on National Forest System lands in Colorado. In the course of the inventory, areas with naturally occurring acid rock drainage were also noted. Approximately 18,000 abandoned mine-related features were inventoried, including about 900 features that are considered significant enough environmental problems to warrant further investigation. Water quality data, such as pH and conductivity were gathered at all features where water was present, such as draining adits, seepage at the toe of dumps and tailings, and standing water in shafts. Samples were taken where field tests indicated low pH and/or high conductivity, including several areas with naturally occurring acid rock drainage. Samples were analyzed for dissolved and total metals, and for selected anions. All mine locations and data collected by the field geologists were entered on field forms and, subsequently, into a computer database and GIS format. With the information provided by the inventory, the Forest Service, in cooperation with other agencies, has been able to prioritize abandoned mines for reclamation. In most cases, cleanup is approached on a watershed basis. Mines in priority watersheds have been selected for reclamation first. Watersheds where studies prerequisite to cleanup are occurring include the upper Animas River, Willow Creek (tributary to the upper Rio Grande), Chalk Creek (tributary to the upper Arkansas River), the Uncompahgre River, and the Alamosa River. During the inventory, evidence of naturally occurring water quality degradation was found in areas where little or no evidence of mining activity exists. These areas include the upper Alamosa River, the Middle Fork of Mineral Creek, Peekaboo Gulch, and Handcart Gulch. Water from these natural sources has been found to significantly exceed Colorado water quality standards for several metals. INTRODUCTION The Rocky Mountain Region (Colorado, Wyoming east of the Continental Divide, and the Black Hills of South Dakota) of the USDA Forest Service (USFS) has recently completed an inventory of abandoned mines. The results indicate that there are approximately 19,000 abandoned mine-related features on National Forest System (NFS) lands in the Rocky Mountain Region of the Forest Service. Of these, about 1,200 are considered environmental problems and 4,500 are considered physical hazards. The 1Colorado Geological Survey, Denver, Colorado 2USDA Forest Service, Lakewood, Colorado 1 majority of the problem sites are located in Colorado where 18,382 mine-related features were identified. The USFS and the Colorado Geological Survey (CGS) completed, under a cooperative agreement, an inventory of environmental degradation associated with abandoned and inactive mines on Colorado's NFS lands. In the course of the inventory, areas with naturally occurring acid rock drainage were also noted. The inventory work began in 1991 and was completed in 1998. The driving force behind the project was the Federal Facilities Compliance Program, which is designed to bring federal facilities and lands into compliance with federal environmental laws including the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA); the Resource Conservation and Recovery Act (RCRA); and the Clean Water Act (CWA) among other laws. The USFS Abandoned Mine Land Inventory Project was a “discovery” process. Identification of environmentally degraded sites may lead to preparation of CERCLA documents consistent with the National Contingency Plan (40 CFR 300). In addition, physical hazards related to abandoned mine sites, such as dangerous shaft or adit openings, were also assessed. Discussion of physical hazards is beyond the scope of this paper. METHODS The inventory process began with an office review of existing mining and geologic literature, previous mine inventories, and current and historical maps. Mine locations from these sources were compiled onto a work map. Natural-color, 1:24,000-scale (approximate) aerial photographs were examined to locate potential mine sites not identified by other sources. Water quality information was used to identify streams potentially affected by acid mine drainage or other mine-site contaminants. When the office research process was complete, geologists entered the field with specific mine locations to visit. Additional mines not identified in the literature search were found while performing the field inventory work. Investigated mines were grouped geographically into “inventory areas” that were given identification numbers based on the Universal Transverse Mercator (UTM) coordinate system. An inventory area usually contains one to twenty mine features that can be grouped in relation to geographic features, such as a gulch or hillside. Mine features include adits, shafts, prospect pits, highwalls, quarries, waste rock dumps, tailings, and spoils. All mine features within an inventory area were numbered sequentially. When the feature number is appended to the inventory area number, every mine feature in the inventory has a unique identification number. Using standardized field-data forms, geologists recorded data on numerous physical and geographic characteristics of the mine features. The quality of any water associated with a mine feature was assessed in the field by determining the pH (acidity), specific conductance (dissolved solids), and observable characteristics of the water. Observable characteristics include precipitates and salts in the effluent drainage, opaque or cloudy water, stressed vegetation, and absence of aquatic organisms. This information was used to assign a qualitative “Environmental Degradation Rating” (EDR) to the individual mine feature. Ratings guidelines (Table 1) facilitated consistency in the data set while allowing the field geologists flexibility to consider site-specific conditions such as geology, effluent discharge volume, surface water interactions, precipitation, etc. The numerical pH and conductivity values given in the ratings are merely guides. Measured values can vary depending on the type of geologic terrane and the location of the mine site within the drainage basin. For example, drainage from areas underlain by sedimentary rocks generally has higher conductivity than drainage from igneous terranes. 2 Table 1 Guidelines for assigning Environmental Degradation Ratings (EDR) Rating (EDR) Feature usually displays one or more of the following characteristics: 1=EXTREME • Contamination off-site is severe. • Receiving stream is "dead" or sterile at the mine and downstream. • Effluent has extremely low pH (<4). • Effluent has extremely high conductivity (>1500 microsiemens per centimeter - µS/cm; >1000 µS/cm in alpine areas). • High flows of poor-quality water, relative to the receiving stream. • Abundant precipitate at the mine and in the receiving stream. • Very large dumps or tailings piles with evidence of severe erosion, especially if they have abundant sulfides. 2=SIGNIFICANT • Receiving stream is significantly or obviously adversely affected, but not "dead" or sterile. • Effluent has low pH (<5). • Effluent has high conductivity (>1000 µS/cm; >500 µS/cm in alpine areas). • Moderate flows of poor-quality water, relative to the receiving stream. • High flows of moderate-quality water, relative to the receiving stream. • Moderate to abundant precipitate at the mine and/or in the receiving stream. • Large sulfide-rich dumps or tailings piles with evidence of moderate erosion. • Large dumps with sparse or no sulfides, but evidence of significant erosion. 3=POTENTIALLY • Evidence of degraded water quality, but serious effects are not obvious SIGNIFICANT or detected. • Effluent has low pH (<5.5). • Effluent has moderate conductivity (>600 µS/cm; >200 µS/cm in alpine areas). • Poor-quality water with low or no flow (standing water). • Moderate flows of moderate-quality water, relative to the receiving stream. • Minor amounts of precipitate. • Very large dumps with little or no evidence of erosion and sparse or no sulfides. • Small and moderate-sized sulfide-rich dumps or tailings piles with evidence of moderate erosion. 4=SLIGHT • Effluent with slightly acidic pH (<6.5). • Effluent with slightly elevated conductivity (>400 µS/cm; >100 µS/cm in alpine areas). • Sparse or no precipitate. • Small to moderate-sized sulfide-rich dumps or tailings piles with little evidence of erosion. 5=NONE • No effluent. • Effluent of high quality water. • Small dumps distant from surface water with little or no evidence of erosion. 3 Water draining from areas of intensely hydrothermally altered rocks often has lower pH and may show elevated conductivity. Sites located topographically higher within a drainage basin may produce water that has relatively lower conductivities because of shorter residence times in the country rock. Water at these sites may still be degraded even though they exhibit lower conductivities than sites topographically lower in the drainage basin. Waters can also have significant concentrations of metals with near neutral pH values. Observable characteristics of the water and the mine site give important clues to the quality of the water in addition to pH and conductivity measurements. There were some necessary limits on the types of mine features included